1. Field of the Invention
The present invention relates to a method and apparatus for producing twine from thermoplastic materials. More particularly, the twine is made from a bundle of oriented thermoplastic monofilaments which is bound by a thermoplastic material, which extends around the bundle in a spiral form.
2. Prior Art
U.S. Pat. No. 3,415,919 to Kippan discloses a method of producing a twine by extruding a thermoplastic material in a spiral on to a untwisted bundle of oriented thermoplastic monofilaments. According to Kippan prior art fiber twine of necessity is twisted in order to transmit stress from one fiber to another and develop the continuity of tensile strength of the cord. The patentee notes that monofilament twine prior to his development was twisted so that the monofilaments contained themselves by intertwined helixes to form a unit twine or cord.
Kippan notes several disadvantages of twisted twine. The primary disadvantage is a weaker twine, since the helixes are able to assume only a portion of an applied tensile load, whereas in a parallel alignment of monofilaments each filament is able to fully bear an applied tensile load. In theory, at least the tensile strength of a twine having all of the monofilaments aligned is the sum of the tensile strengths of each filament (in practice tensile strength is slightly less).
Another disadvantage of the twisted cord according to Kippan is that it tends to untwist when it is slack, forming snarls which can cause problems in mechanical hay balers.
Other prior art of interest in U.S. Pat. Nos. 3,446,002 to Kippan (monofilament twine), 3,769,787 to Rosenstein (yarn composed of twisted or untwisted core of filaments is wrapped helically, both clockwise and counter clockwise), 3,577,873 and 3,851,457 to Waters (a yarn composed of a set false twist and reversing helical binding).
It has been found that twine or cord such as described by Kippan having parallel monofilaments bound by an extruded binder, also is disadvantageous in mechanical balers. The straight monofilaments can be deformed in the knotting operations such that individual fibers break out between the spiral binding. The structure becomes less homogeneous in the knotting step and a protruding monofilament may catch in the mechanism.